The invention relates to data communication systems.
In a data communication system or network, a number of communicating devices are connected together, for example in a ring, and data is passed around the network, the data being in digital form and being generated or regenerated at each device on the network under the control of a local clock. One of the problems which arises is that the clock generated at each device may not be precise and may exhibit a (random) phase variation or jitter. Jitter can be considered as the difference between the phase of a perfectly stable clock at the nominal clock frequency and the clock frequency embedded in the data stream. The jitter amplitude is a measure of how far the phase of the clock embedded in the data stream deviates from the phase of the stable clock; the jitter frequency is a measure of how fast the relative phase changes. Jitter can be defined in several ways, for example in time or frequency domains, and the definition of jitter may vary according to the type of network concerned. In most cases the applicable network standard (such as IEEE 802.5 token ring; ANSI X3T9.5 FDDI. etc) will explain the precise definitions of jitter used in their systems.
When data is received at a device, the device has to be able to decode the data despite the phase jitter which exists. In an existing data communications network it is therefore necessary to measure the jitter tolerance of the network, that is how much jitter the network can tolerate before an error condition will be indicated and data can no longer be reliably decoded.
At present, the jitter tolerance of an installed data communications network can only be determined in terms of the error rate of data frames sent around the network (usually a ring). This technique gives no information about the margins or tolerances present in the network and hence the risk of introducing an unacceptable error rate by extending the network.
During production testing of individual items of data communications equipment, the jitter tolerance is measured by an additional test, which increases test time and requires complex measuring equipment to monitor the bit error rate of the equipment under test. Furthermore, in the particular case of communications systems based on a chain of repeaters, for example a ring-based local area network, the jitter is not due simply to the individual items of data communications equipment but is a complex function also of the properties of the installed cable and is practically impossible to predict from measurements made on the individual network components.